An automatic slack adjuster provides for periodic adjustment of the release position of a screw-threaded piston rod of the brake cylinder, to compensate for brake shoe wear and thereby maintain a normal preset clearance between the brake shoes and the braking surface on the wheel when the brakes are in a released state. The screw-threaded piston rod is arranged with a coupling nut and a displacement nut, all of which are axially positionally adjustable relative to each other.
Since the automatic slack adjuster referred to herein is of conventional type and the details thereof are not considered essential to an understanding of the present invention, only a brief statement describing the manner of operation thereof will be given. When a brake application is effected, if the amount of piston and piston rod travel exceeds a certain increment of such travel predetermined to be compatible with the amount of travel needed to bring the brake shoes into contact with the braking surfaces of the wheel, resulting relative rotation between the displacement nut and the coupling nut cause an axial positional adjustment of the piston rod relative to the piston to compensate for the excessive travel of the piston rod and piston. This axial positional adjustment of the piston rod accumulates until the shoes are worn to a degree at which they must be replaced with new shoes. In order to provide the normal preset clearance between the new shoes, which are of full thickness, and the wheel surfaces, the brake rod must be axially reset, that is, retracted from its final adjusted position (the point at which the shoes are totally worn) to an initial position (the point at which the shoes were new).
Resetting of the piston rod, when the brake shoes are replaced, is normally accomplished manually. Some of the presently known arrangements for manually resetting the piston rod include a self-locking worm gear or other suitable gear by which the coupling and displacement nuts may be manually rotated from the exterior of the cylinder casing for retracting the piston rod to the initial position. Another presently known arrangement for resetting the piston rod includes a two-section cylinder casing, one section of which is fixed, and the other section of which, after loosening a locking device, is manually rotatable relative to the fixed section to cause rotation of the coupling and displacement nuts for resetting the piston rod.
A two-section cylinder casing is not desirable. Moreover, by nature the structures, as above described, involving a self-locking worn gear or the two-section cylinder casing with the lock mechanism therefor are complex and costly.